The present invention relates to an objective lens for an optical information recording/reproducing apparatus configured to record information to and/or reproduce information from a plurality of types of optical discs based on different standards, and to an optical information recording/reproducing apparatus on which such an objected lens is mounted.
There exist various standards of optical discs, such as DVD (Digital Versatile Disc) and BD (Blu-ray Disc), differing in recording density, protective layer thickness, etc. Therefore, an objective optical system mounted on the optical information recording/reproducing apparatus is required to have a compatibility with a plurality of types of optical discs. In this case, the term “compatibility” means to guarantee realizing information recording and information reproducing without the need for replacement of components even when the optical disc being used is changed. Incidentally, in this specification, the “optical information recording/reproducing apparatuses” include apparatuses for both information reproducing and information recording, apparatuses exclusively for information reproducing, and apparatuses exclusively for information recording.
In order to have the compatibility with the plurality of types of optical discs based on the different standards, the objective optical system needs to correct the spherical aberration which changes depending on the difference in protective layer thickness (i.e., an optical distance between a recording surface and a surface of a protective layer of an optical disc) between the optical discs and to form a suitable beam spot in accordance with the difference in recording density between the optical discs, by changing the numerical aperture NA of the objective optical system. In general, the size of the beam spot becomes smaller, as the wavelength of a laser beam becomes shorter. For this reason, the optical information recording/reproducing apparatus is configured to selectively use one of a plurality of types of laser beams having different wavelengths in accordance with the recording density of the optical disc being used. The optical information recording/reproducing apparatus uses, for example, light having the wavelength of approximately 660 nm (i.e., so-called red laser light) for information recording or information reproducing for DVD, and light having the wavelength of approximately 405 nm (i.e., so-called blue laser light) for information recording or information reproducing for BD.
Japanese Patent Provisional Publications No. 2000-81566A (hereafter, referred to as JP2000-81566A), No. 2004-265573A (hereafter, referred to as JP2004-265573A) and No. 2004-326862A (hereafter, referred to as JP2004-326862A) disclose optical information recording/reproducing apparatuses having the compatibility with a plurality of types of optical discs. Specifically, JP2000-81566A discloses an optical information recording/reproducing apparatus having the compatibility with both of the standards of CD (Compact Disc) and DVD. Each of JP2004-265573A and JP2004-326862A discloses an optical information recording/reproducing apparatus having the compatibility with both of DVD and BD. As disclosed in each of JP2000-81566A, JP2004-265573A and JP2004-326862A, an objective lens for the optical information recording/reproducing apparatus is provided with a diffraction structure on one of surfaces of the objective lens to achieve the compatibility. The diffraction structure has a plurality of annular zones concentrically formed about an optical axis of the objective lens. The diffraction structure of this type has a plurality of regions in which diffraction structures having different blazed wavelengths are respectively formed in order to enhance the diffraction efficiency for each of a plurality of laser beams based on the different standards. Specifically, the diffraction structure has a first region contributing to convergence of each of the plurality of laser beams onto a recording surface of corresponding one of the plurality of optical discs, and a second region contributing only to convergence of the laser beam for a high recording density optical disc (i.e., DVD in the case of JP2000-81566A, and BD in the case of JP2004-265573A or JP2004-326862A).
Incidentally, when the compatibility with a plurality of optical discs based on different standards is provided for a single objective lens, there is a concern that the quality of a signal is deteriorated by flare light. For example, when CD is used in an optical system disclosed in JP2000-81566A (or when DVD is used in an optical system disclosed in JP2004-265573A or JP2004-326862A), principally two types of flare light including undesired diffraction order light in a first region and undesired light in a second region are caused. In order to suitably perform information recording or information reproducing for each optical disc, it is necessary to reduce the effect caused on a signal by the flare light.
However, regarding the optical information recording/reproducing apparatus disclosed in JP2000-81566A, there is a problem that, when the objective lens approaches CD under focus-servo-control, the undesired diffraction order light in the first region is converged on a recording surface of CD and thereby deteriorates the quality of a signal. On the other hand, regarding the optical information recording/reproducing apparatus disclosed in each of JP2004-265573A and JP2004-326862A, the absolute amount of undesired diffraction order light is small because the diffraction efficiency is increased for each of the plurality of laser beams based on the different standard. However, the optical information recording/reproducing apparatus disclosed in each of JP2000-81566A, JP2004-265573A and JP2004-326862A is designed to use different high orders of diffracted light respectively for the plurality of laser beams in order to enhance the diffraction efficiency. In this case, a problem arises that the height of each step in the diffraction structure becomes large, and therefore processing of a metal mold and molding for the objective lens becomes difficult.